1. Field of the Invention
The present invention relates to paper machine belts and particularly, but not exclusively, to paper machine process belts such as belts for transferring and/or smoothing the paper web within, to and/or from the press section of a paper machine.
2. Discussion of Background Information
Transfer belts are used for carrying a paper web through a portion of a paper machine so as to eliminate open draws in which the paper web is unsupported and is thus likely to break. When the web breaks, the paper machine must be shut down and consequently this constitutes a serious problem to the papermaker. Such transfer belts tend to have a smooth surface which can aid smoothing of the paper sheet and provide an extremely uniform pressure distribution in the nip with no basecloth mark.
The belt surface should also provide for release of the paper web from the belt. In the paper machine, the paper web tends to remain adhered to smooth belt surfaces via a film of water, which forms between the web and the belt. For web release to be achieved, this continuous film of water needs to be broken. Prior art belts, which facilitate ready sheet release, have utilized polymeric coating layers impregnated with a fibrous or particulate material such that the fibers or the particles are exposed on the web-receiving surface of the belt to modify the belt's surface characteristics.
One transfer belt of this type is described in U.S. Pat. No. 5,298,124. Here, the sheet release post transfer is assisted by incorporating particles, which under pressure are compressed into the belt matrix, but on release of pressure at the web release point, stand proud of the belt surface and thus create a temporary roughening of the surface which aids sheet release. The transfer belt includes a woven base structure, which results in pronounced marking of the paper sheet.
U.S. Pat. No. 4,500,588 relates to a conveyor felt including one or more fibrous batt layers needled on a woven support fabric as well as a filling material filling the support fabric and the fiber batt layers with the exception of the surface facing the web. The surface of the felt is calendered. The woven base fabric results in marking of the paper web.
In EP 1127976 a transfer belt includes a base support having a layer of thermoplastic material formed thereon. A batt of fibrous material is located on top of this thermoplastic material. The whole structure is then heated in order to allow the thermoplastic material to migrate to the surface. This produces a polymeric surface with embedded fibers, which can assist with the controlled separation of the paper sheet and the belt. The woven base fabric results in marking of the paper sheet.
In EP 1085124 a transfer belt includes a polymeric resin matrix mixed with a fibrous or particulate material. One of the matrix or the fibrous/particulate materials is hydrophobic. The paper web-receiving face of the transfer belt is polished to expose the fibers/particles. This arrangement suffers from the drawback that the fibers/particles are unlikely to be uniformly mixed with the resin or uniformly orientated within the resin. Thus, on polishing, the degree of exposure of the fibrous/particles at the web-receiving surface will be non-uniform. Again, the woven base fabric results in marking of the paper sheet.